The term "oil shale as used in the industry refers to a sedimentary formation comprising marlstone deposits with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. The formation containing kerogen is called "oil shale" herein and the liquid product produced upon decomposition of kerogen is called "shale oil".
In a preferred practice of the method described herein, the method is utilized for refining shale oil produced from in situ retorting of oil shale. An in situ shale retort can be formed by many methods, such as the methods disclosed in U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; and 4,043,598, all of which are incorporated herein by this reference.
The process can also be practiced on shale oil produced by other methods of retorting. Many of these methods for shale oil production are described in Synthetic Fuels Data Handbook, compiled by Dr. Thomas A. Henrickson, and published by Cameron Engineers, Inc., Denver, Colo. For example, other processes for retorting oil shale include those known as the TOSCO, Paraho Direct, Paraho Indirect, N-T-U, and Bureau of Mines, Rock Springs, processes.
Kerogen is considered to have been formed by the deposition of plant and animal remains in marine and nonmarine environments. Its formation is unique in nature. Alteration of this deposited material during subsequent geological periods produced a wide variety of organic materials. Source material and conditions of deposition were major factors influencing the type of final product formed.
Kerogen samples, found in various parts of the world, have nearly the same elemental composition. However, kerogen can consist of many different compounds having differing chemical structures. Some compounds found in kerogen have the structures of proteins while some have structures of terpenoids, and others have structures of asphalts and bitumens.
Shale oils are generally high molecular weight, viscous organic liquids, of predominantly hydrocarbonaceous oxygen, nitrogen and sulfur-containing organic compounds produced from oil shale. The shale oils are of varying linear, branched cyclic, aromatic hydrocarbon and substituted hydrocarbon content with high pour points, moderate sulfur content, large amounts of metallic impurities, especially arsenic, and relatively high nitrogen content.
The shale oil produced from an oil shale formation can vary between strata within the oil shale formation. The nitrogen content of shale oil can also vary dependent upon the geographical location of the oil shale deposit from which the shale oil is produced. Such a variance in nitrogen content in different geographical locations can be atttibuted to differences in the environment during the time of the deposition of the organisms which, upon lithification, become oil shale. Such a variance can also be attributed to the different types of organisms in the separate geographical locations which were deposited to form the organic substance in the oil shale and any organisms within the formed deposit layer which acted upon such deposited material to provide the kerogen within the oil shale formation. Furthermore, the nitrogen content of shale oil may vary according to the process and operating variables used to produce it.
The nitrogen content in shale oil is attributable to basic nitrogen-containing compounds and non-basic nitrogen-containing compounds. The relative percentages of the basic and non-basic nitrogen compounds comprising the total nitrogen content of a shale oil varies depending upon the particular shale oil but typically are in the ranges of 60% to 70% basic nitrogen-containing compounds and 30% to 40% non-basic nitrogen-containing compounds.
The nitrogen content of shale oil is generally up to about two percent by weight. For example, the average nitrogen content of shale oil recovered by in situ retorting of oil shale from the Piceance Creek Basin of Western Colorado is on the order of about 1.4% by weight. This is very high when compared with the nitrogen content of crude petroleum which is typically no more than about 0.3% by weight.
The presence of nitrogen in shale oil presents many problems in that the nitrogen can interfere with refining and the transportation and use of shale oil. Deleterious effects brought about by the presence of nitrogen in shale oil are decreased catalyst life in hydrogenation, reforming, hydrocracking and catalytic cracking reactions, decreased chemical stability of products, and decreased color stability of products.
Another problem with the presence of nitrogen in shale oil is that it is undesirable to transport nitrogen-containing shale oil through pipelines which are also used for transporting petroleum products because of possible contamination of such products with residual nitrogen-containing shale oil in the pipeline. Generally such petroleum products contain a very low nitrogen content. The relatively high nitrogen content in the shale oil can pollute the pipelines making them undesirable and uneconomical for transporting such low nitrogen-containing petroleum products. In addition, a high nitrogen content in shale oil can cause clogging of pipelines due to self-polymerization brought about by the reactivity of the nitrogen-containing compounds. Due to the basicity of the nitrogen-containing compounds in shale oil, some corrosion can occur, thus damaging a pipeline used to transport shale oil.
Product stability is a problem that is common to many products derived from shale oil with the major exception of the asphalt cut and those products that have undergone extensive hydrotreating. Such instability, including photosensitivity, is believed to result primarily from the presence of nitrogen-containing compounds.
It is, therefore, desirable to reduce the nitrogen content of shale oil to increase the utility, transportability, and stability of the shale oil and the products derived from such shale oil.
Conventionally, nitrogen removal in shale oil has been achieved by hydrogenation processes, extraction processes or a combination of both processes.
In extraction processes, the shale oil is contacted with an extraction agent, usually an immiscible solvent capable of selectively extracting nitrogen-containing compounds. As illustrative, U.S. Pat. No. 4,272,361 to Compton discloses a method for reducing the nitrogen content of shale oil by contact with an aqueous solution comprising an active solvent for nitrogen-containing compounds and sufficient water to provide phase separation. The active solvent is selected from the group consisting of organic acids and substituted organic acids.
Extraction processes are useful in extracting a portion of the nitrogen-containing compounds from shale oil. However, the selectivity of these processes is insufficient to reduce the nitrogen content to a level wherein the shale oil can undergo further refinement, such as catalytic cracking, without extracting a significantly large portion of the non-nitrogen-containing compounds. This leads to a low oil recovery.
In hydrogenation processes, also referred to as hydrotreating, the shale oil is heated in the presence of hydrogen gas under extreme pressure. This results in a very large consumption of hydrogen gas. For example, reduction of the nitrogen content of shale oil to about 500 ppm may require a partial hydrogen pressure of about 2,000 psi or more at a temperature of from about 760.degree. F. to 790.degree. F. and from about 0.5 to about 1.0 liquid hourly space velocity (LHSV). Hydrogen consumption of about 2,500 standard cubic feet per barrel results.
Combination processes inclucing extracting extraction and hydrotreating have also been disclosed. The object of these processes is to provide a method for reducing the hydrogen consumption that results from upgrading high nitrogen oil feed stocks.
For example, in U.S. Pat. No. 4,159,940 to Smith, there is disclosed a process wherein a high nitrogen syncrude feed is contacted with at least one acid selected from the group consisting of sulfuric, phosphoric and hydrochloric acids to produce a first phase low in nitrogen and a second phase high in nitrogen. The second phase then undergoes severe hydrotreating and the first phase undergoes mild hydrotreating.
U.S. Pat. No. 4,261,813 also to Smith improves the above process by removing the acid solvent from the high nitrogen phase to produce a high-nitrogen extract oil which is passed to a hydrogen-producing plant to supply hydrogen for hydrotreating. The low nitrogen first phase is hydrotreated at mild conditions.
U.S. Pat. No. 4,287,051 to Curtin discloses a process wherein a high nitrogen feed oil is separated into a first portion and a remaining highly viscous portion. Nitrogen compounds are extracted from the first portion with an acid solvent to produce a low nitrogen raffinate and a high nitrogen extract. The acid solvent is then recovered from the extract to produce a high-nitrogen extract oil. The highly viscous portion and the high-nitrogen extract oil are partially oxidized to produce hydrogen which is used to mildly hydrotreat the low nitrogen raffinate.
These combination processes demonstrate an attempt to reduce the hydrogen consumption of hydrotreating shale oil by incorporating a liquid extraction step. However, to maximize oil recovery, both the low nitrogen phase and the high nitrogen phase resulting from the extraction must be hydrotreated. This results in separate hydrotreating which is expensive. The alternative is to not hydrotreat the second high nitrogen phase. However, this reduces oil recovery.